Several conventional logging while drilling (“LWD”) calipers currently exist. Of these, the ultrasonic calipers generally offer a good and direct measurement. This caliper may offer precision azimuthal hole shape definition in suitable environments and is generally not restricted to specific mud types.
For robust ultrasonic caliper measurements, it is desirable that acoustic properties of the mud are known or can be derived, and that there is sufficient contrast in acoustic impedance between the mud and formation wall. Unfortunately, these boundary conditions associated with mud sometimes cannot be calculated.
Meanwhile, other conventional azimuthal LWD calipers have a limited depth of investigation range and are susceptible to mud types, and in particular, high barite muds. In other caliper options, density and neutron based measurements can be used to derive non-directional caliper information. These LWD calipers may have the advantage that they are feasible in sliding mode, but as with most neutron log measurements, the neutron caliper is sensitive to mud properties and eccentering. The azimuthal information from the density caliper cannot be obtained when the tool is sliding as there is generally one sensor. The term sliding refers to non-rotation of the bottomhole assembly, such as occurs when drilling with a mud motor, tripping into a well, or tripping out of a well.
Other calipers include propagation resistivity tools. Such resistivity tools used as calipers may offer good quality caliper information in water based muds and can be derived from conventionally acquired data as a byproduct of an inversion. However, these resistivity tools as well as the other conventional calipers mentioned above are limited in that they cannot provide consistent and dependable high quality caliper measurements across various types of conditions, including different mud types and during washout conditions while providing measurements at different depth ranges.